1. Field of the Invention
The present invention relates to a fuel supply apparatus for an automotive engine and particularly relates to an automotive fuel supply apparatus for reducing engine fuel consumption.
2. Description of the Related Art
FIG. 9 is a schematic diagram showing a general overview of a conventional automotive fuel supply apparatus.
In FIG. 9, a fuel pump 1 is disposed inside a fuel tank 2 and is connected to a fuel injection valve 4 of an engine 5 by a fuel distribution line 3. The fuel pump 1 is provided with: a pump main body portion 1a; an electric motor portion 1b for driving the pump main body portion 1a; and a check valve 1c for improving engine starting by keeping a fuel system including the fuel distribution line 3 charged with fuel when the engine 5 is stopped. Furthermore, a switching relay 16 is controlled by a pump control portion 13a of an engine control apparatus 13 described below such that a voltage from a battery 15 is applied to the motor portion 1b when the engine 5 is running, and an electrical connection between the battery 15 and the motor portion 1b is shut off when the engine 5 is stopped.
The fuel injection valve 4 is connected to an intake air manifold 6 of the engine 5, is activated and controlled by the engine control apparatus 13, and supplies fuel to the engine 5.
A fuel pressure regulator 7 is constructed such that a spring chamber 8 and a pressure regulating chamber 9 are partitioned by a diaphragm 10. A regulator spring 8a is disposed inside the spring chamber 8 so as to press on the diaphragm 10. The pressure regulating chamber 9 is provided with: a discharge orifice 9a; and a valve body 9b mounted to the diaphragm 10, for opening and closing the discharge orifice 9a. The spring chamber 8 communicates with the intake air manifold 6 upstream from the fuel injection valve 4 through a first branch line 11a, and the pressure regulating chamber 9 communicates with the fuel distribution line 3 through a second branch line 11b. In addition, the pressure regulating chamber 9 communicates with the fuel tank 2 through the discharge orifice 9a and a return line 12.
The engine control apparatus 13 is provided with a pump control portion 13a and a fuel computing control portion 13b, a required quantity of fuel supply being calculated by the fuel computing control portion 13b to control the valve opening time of the fuel injection valve 4 based on the quantity of intake air which the engine 5 has drawn in after making a pressure difference upstream and downstream from the fuel injection valve 4 constant. Here, a “D-Jetronic” method is adopted as the method by which the fuel computing control portion 13b calculates the required quantity of fuel supply to the engine, the required quantity of fuel supply being calculated based on pressure inside the intake air manifold 6 measured directly by an intake air manifold pressure detector 14.
Moreover, an air flow sensor may also be mounted to the intake air manifold 6 instead of the intake air manifold pressure detector 14, the required quantity of fuel supply being calculated based on the quantity of intake air per unit time in the engine 5 detected by the air flow sensor (an “L-Jetronic” method).
In the conventional automotive fuel supply apparatus constructed in this manner, fuel conveyed under pressure by the fuel pump 1 is supplied to the fuel injection valve 4 through the fuel distribution line 3. Fuel fed into the fuel distribution line 3 is prevented from flowing back into the fuel tank 2 by the action of the check valve 1c. Thus, the fuel distribution line 3 is always charged with fuel, even when the engine 5 is stopped.
The pressure inside the intake air manifold 6 is introduced into the spring chamber 8 through the first branch line 11a, and the fuel inside the fuel distribution line 3 is introduced into the pressure regulating chamber 9 through the second branch line 11b. When the pressure of the regulator spring 8a and the pressure inside the intake air manifold 6 are greater than the pressure inside the pressure regulating chamber 9, the diaphragm 10 is pressed toward the pressure regulating chamber 9 and the valve body 9b blocks the discharge orifice 9a. When the pressure of the regulator spring 8a and the pressure inside the intake air manifold 6 are less than the pressure inside the pressure regulating chamber 9, the diaphragm 10 is pressed toward the spring chamber 8, separating the valve body 9b from the discharge orifice 9a and permitting fuel to flow back through the discharge orifice 9a and the return line 12 to the fuel tank 2. In other words, any fuel supplied to the fuel distribution line 3 other than the fuel supplied to the engine 5 from the fuel injection valve 4 is returned through the return line 12 to the fuel tank 2. Thus, the pressure difference upstream and downstream from the fuel injection valve 4 is kept constant. This pressure difference can be set arbitrarily by adjusting the elastic force of the regulator spring 8a. 
Now, there is a difference of approximately two orders of magnitude (100 times) in the fuel consumption of the engine 5 per unit time when idling and when at maximum output. Generally, this means that the fuel pump 1 is set to a performance at which a sufficient fuel supply can be maintained at maximum output and is constantly operated at this maximum-output setting. Thus, electric power generated in an alternator (not shown) by driving the engine 5 is consumed wastefully by the fuel pump 1 operating at this maximum-output setting, resulting in the consumption of fuel being increased.
When operating conditions are such that the service region of the engine 5 is only in a low-output region, such as in the 10-mode and 15-mode tests defined by the Japanese Ministry of Land, Infrastructure, and Transport, electric power losses due to the fuel pump 1 are particularly large, accounting for approximately three to four percent in a conventional 1500 cc passenger car.
Next, reduction of fuel pump losses in conventional fuel pump control will be explained with reference to FIG. 10. Moreover, FIG. 10 is a graph explaining the performance of the fuel pump, solid lines representing plots of pump discharge pressure P against pump discharge flow rate Q (P versus Q) and dotted chain lines representing plots of pump discharge pressure P against motor current I (P versus I). In FIG. 10, plots of P versus Q when a drive voltage E of the motor portion 1b is 14 V, 12 V, and 10.5 V, respectively, and plots of P versus I when the drive voltage E of the motor portion 1b is 14 V and 12 V, respectively, are shown.
First, if the pressure is controlled by the fuel pressure regulator 7 so as to be 0.45 MPa, for example, when the drive voltage is 14V, the fuel pump 1 operates with point A in FIG. 10 as an operating point, discharging 90 l/h of fuel. At this time, the motor current I is at point G on the plot of P versus I, consuming an electric current of 5.4 A, making a consumption of approximately 76 W when converted to electric power.
Generally, automotive engines 5 are multicylinder, and as engine output increases, a plurality of fuel injection valves 4 may open simultaneously, but the number of fuel injection valves 4 which open simultaneously is set to two so that the maximum performance of the fuel pump 1, which introduces losses, does not become needlessly large.
For example, in a 1500 cc four-cylinder engine 5, displacement is 375 cc per cylinder, making the quantity of fuel required for the cylinders to generate maximum torque approximately 0.055 cc, assuming an air-fuel ratio of 12:1. At the same time, if an engine rotational frequency generating maximum output is 6,000 rpm, then injection occurs fifty times per second, requiring 2.75 cc of fuel every second. Consequently, for four cylinders, 11 cc of fuel is required every second. In other words, when operating such that only one fuel injection valve 4 is opened, the maximum required fuel demanded by the engine is approximately 40 l/h. Furthermore, it is necessary for the fuel injection valves 4 to inject 0.055 cc of fuel within five milliseconds in each injection, but when injection capacity is low, injection may take longer than five milliseconds.
Thus, in an instant (when two fuel injection valves 4 open simultaneously), a discharge capacity of approximately 80 l/h is demanded of the fuel pump 1, being twice the maximum required fuel demanded by the engine described above.
In FIG. 10, the point where two fuel injection valves 4 open simultaneously is point B, and the point where one fuel injection valve 4 opens is point C which is half of point B. In other words, when two fuel injection valves 4 open simultaneously, the quantity of flow between point A and point B is discharged from the fuel pump 1 wastefully, consuming energy given by the product of that quantity of flow and the fuel pressure. In addition, when one fuel injection valve 4 opens, the quantity of flow between point A and point C is discharged from the fuel pump 1 wastefully.
When all of the fuel injection valves 4 are closed, the quantity of flow between point A and point D, representing complete discharge, returns to the fuel tank 2, consuming all 76 W of electric energy wastefully.
Thus, it has been proposed that the wasted portion in the quantity of discharge from the fuel pump 1 be reduced by controlling the electric power supplied to the fuel pump 1 in response to the service region of the engine 5.
In a conventional fuel supply apparatus proposed as an improvement, as shown in FIG. 11, a switching relay 16A is controlled such that the voltage (14 V) from the battery 15 is supplied to the motor portion 1b directly when output from the engine 5 is at a maximum, and the voltage from the battery 15 is supplied to the motor portion 1b through a resistor 17 when operating such that only one fuel injection valves 4 is being opened. Here, the resistor 17 is set such that the drive voltage for the motor portion 1b is 12 V, for example, in other words, such that the operating point of the fuel pump 1 is point E.
Because the conventional fuel supply apparatus proposed as an improvement is designed to operate such that the drive voltage for the motor portion 1b is switched between 14 V and 12 V by the switching relay 16A, a loss corresponding to the quantity of flow between point A and point E is recovered when only one fuel injection valve 4 is being opened.
However, in the conventional fuel supply apparatus proposed as an improvement, the quantity of flow between point C and point E when one fuel injection valve 4 is open, and between point D and point E when the fuel injection valves 4 are closed is still discharged wastefully by the fuel pump 1, making the recovery of losses insufficient.
As can be seen from the fuel pump characteristics (P versus Q characteristics and P versus I characteristics) in FIG. 10, even if the quantity of discharge from the fuel pump 1 is reduced by forty percent, only a twenty-five percent reduction is achieved in electric energy.
Furthermore, because the voltage (14 V) from the battery 15 is dropped to 12 V by the resistor 17 before being supplied to the motor portion 1b, one problem has been that losses due to Joule heat at the resistor 17 arise instead, preventing sufficient reductions in conventional electric energy, and in turn reductions in fuel consumption, from being achieved.
Thus, in order to eliminate losses resulting from Joule heat in the resistor 17, as shown in FIG. 12, it is conceivable for the drive voltage for the motor portion 1b to be switched to reduce the mean current by switching the large current flowing from the battery 15 to the motor portion 1b using a transistor 18, a method also known as “chopping”. However, there are problems with this chopping method such as requiring the use of a large transistor 18 which generates heat, and increasing the scale of circuitry to control the transistor 18, thereby creating a burden when mounted to the engine control apparatus 13. Another problem has been that undesirable emission of radio waves is generated by chopping of the motor current, adversely affecting electronic devices such as radios, etc.
In methods controlling the voltage supplied to the motor portion 1b such as those described above, it is necessary to increase the discharge performance of the fuel pump 1 suddenly when two fuel injection valves 4 are opened simultaneously. However, even if the voltage supplied to the fuel pump 1 is increased swiftly, the rotational frequency of the motor cannot rise rapidly due to the inertial force of the motor portion 1b. As a result, a delay corresponding to a rise time constant of the motor portion 1b occurs. Then, if the quantity of discharge from the fuel pump 1 does not meet the injection quantity demanded by the fuel injection valves 4, pressure inside the fuel distribution line 3 drops due to this delay to an intermediate point F between the plot of P versus Q for the drive voltage of 14 V and the plot of P versus Q for the drive voltage of 12 V. Because the injection quantity is controlled by controlling the valve opening time of the fuel injection valves 4 under conditions where the pressure inside the fuel distribution line 3 is controlled so as to be constant by the fuel pressure regulator 7, if the pressure inside the fuel distribution line 3 drops to point F, the injected quantity of fuel becomes deficient by an amount corresponding to that drop and irregular combustion may arise, giving rise to problems such as knocking, etc.
For that reason, even when the engine should normally operate at point E, the operating range must be expanded to allow operation at point A, preventing sufficient loss reductions from being achieved.